Conventional lubricating fluids for power transmissions include a fluid comprised mainly of a mineral oil, a fluid comprised mainly of a naphthenic synthetic oil or poly-.alpha.-olefinic synthetic oil, and a fluid comprised of a mixture of two or more mineral oils or synthetic oils. Of these conventional lubricating fluids, the fluid comprised mainly of a naphthenic synthetic oil has been extensively studied because of its higher performance in power transmissions
It is known that a hydrogenation product of .alpha.-alkylstyrene dimer exhibits the highest power transmission performance of all presently commercially available synthetic lubricating fluids. A representative example of such a synthetic lubricating fluid is "Santotrack" manufactured by the Monsanto Chemical Company and disclosed in, e.g., Japanese Patent Laid-Open No. 7664/1972. It has been suggested that the traction coefficient of a traction fluid can be increased if the fluid comprises a mixture of at least two components rather than a single component. Therefore, attempts have been made to prepare a synthetic fluid by mixing the above-mentioned hydrogenation product of .alpha.-alkylstyrene dimer or naphthenes, such as dimer, trimer or codimer which is produced from styrene, alkylstyrene or its derivatives, with paraffinic hydrocarbons. Such a lubricating fluid, is disclosed in Japanese Patent Publication No. 35763/1972.
However, the commercially available hydrogenation product of .alpha.-alkylstyrene dimer has the disadvantage in that the raw materials used for its production are rather expensive. Furthermore, the present trend in the industry is to develope smaller and more efficient power transmissions. In order to fully utilize the performance of newly developed power transmissions it is necessary to further improve the traction coefficient of the synthetic lubricating fluid used in these transmissions.
The term "traction coefficient" as used herein is defined as the ratio of the tractional force which is caused by slipping at the contact points between two rotating members which are in contact with each other in a power transmission of the rolling friction type to the normal load.
In recent years there has been progress in the development of continuously variable transmissions in the automobile industry. In such a transmission the higher the traction coefficient of the lubricating fluid the larger the force transmitted. This allows a reduction in size of the entire device with a corresponding reduction in the emission of polluting exhaust gas. Therefore, there is a strong demand for a lubricating fluid exhibiting the highest possible traction coefficient.
In general, it was believed that mixing at least two base oils could provide a fluid having a desired traction coefficient because of the additive effect discussed hereinafore. However, this additive effect does not exist with all base oils. For example, it is known that no additive improvement in performance is obtained when a naphthenic synthetic oil is blended with about 5 to 50% by weight of a viscosity index improver such as polymethacrylate.
In other words, the performance of power transmissions of automobiles or tractors can be remarkably enhanced if a synthetic lubricating fluid having a high traction coefficient is combined with another component which does not exhibit a high traction coefficient when used alone but exhibits a noticeable synergistic effect when used together with the fluid. However, such a synthetic lubricating fluid has not yet been developed.
The present inventors have made extensive and intensive studies with a view to developing a fluid having a high traction coefficient for use as a lubricating fluid in power transmissions. The present inventors have found that the addition of a specific amount of a branched poly-.alpha.-olefin to a synthetic naphthenic ester can meet the above-mentioned requirements. The present invention is based on this finding.